dlm: use per-attribute show and store methods

To simplify the configfs interface and remove boilerplate code that also
causes binary bloat.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Teigland <teigland@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>